This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 2002-28656 filed in Korea on Mar. 23, 2002; the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a ferroelectric memory, and more particularly, to a high-density ferroelectric memory which is thermally stable.
2. Description of the Related Art
In a ferroelectric memory, the size of a domain is small and a polarization direction can change quickly, and thus it has attracted much attention and is seen as a potential next-generation memory. Ferroelectric Random Access Memory (FeRAM) is an example of conventional memory using a ferroelectric material.
FeRAM stores information using a capacitor formed of ferroelectric materials, which may be spontaneously polarized at a temperature lower than a predetermined temperature and may have the magnitude and direction of polarization, which can be controlled and reversed, respectively, by an external electric field. Since a ferroelectric capacitor is on the scale of microns in size, it does not have a serious problem regarding the thermal stability of a domain at which dielectric polarization occurs. However, it has limitations in size, recording and reproducing speed, and information integration density.
To realize a memory having a small size, high integration density, and improved recording and reproducing speed, a memory which can record and reproduce information using a probe has been researched.
However, a probe-type storage medium using a ferroelectric material has a fatigue phenomenon, in which storage capacity decreases as the number of read/write cycles increases over a predetermined value, and an aging phenomenon, in which polarization gradually disappears as time lapses. The time for which a ferroelectric memory can store information without switching is referred to as a retention time. The retention time increases as resistant ability to the aging phenomenon of polarization increases.
To increase the reliability of information storage in a memory using a ferroelectric material, it is necessary to hinder the fatigue phenomenon and increase the retention time. Accordingly, a method for increasing the thermal stability of bits is desired for the latter aspect.
To solve the above-described problems, it is an object of the present invention to provide a ferroelectric memory which can increase the reliability of information storage by increasing the thermal stability of bits.
To achieve the above object of the present invention, there is provided a ferroelectric memory including a lower electrode, and a ferroelectric layer formed on the top surface of the lower electrode such that a domain having a dielectric polarization is set as a bit. The thickness of the ferroelectric layer is not greater than the size of the bit.
The thickness xe2x80x9cdxe2x80x9d of the ferroelectric layer satisfies Formula (1) with respect to the area A of the bit.                     d        ≤                  2          ⁢                                    A              /              π                                                          (        1        )            
Accordingly, the present invention forms nano-sized ferroelectric domains penetrating a ferroelectric layer in a ferroelectric memory, thereby increasing the thermal stability of bits. Consequently, reliability of information storage can be improved.